Fuel Cells

Introduction

fuel_cell2A fuel cell converts the chemical energy from a fuel into electricity, heat and water through a chemical reaction with oxygen. Hydrogen is the most common fuel and is produced from the steam methane reforming of natural gas, but for greater efficiency hydrocarbons can be used directly, be it natural gas, gasoline, or alcohols like methanol.

Since fuel cells rely on an electrochemical process and not combustion, emissions from fuel cells are significantly lower than emissions from even the cleanest fuel combustion processes.

Fuel Cell Basics

fuel_cellThere are many types of fuel cells, but they all consist of an anode, a cathode and an electrolyte that allows charges to move between the two sides of the fuel cell. Electrons are drawn from the anode to the cathode through an external circuit, producing direct current electricity. A single fuel cell generates about 0.7 volts, just about enough to power a single light bulb. When cells are stacked in series the output increases, resulting in fuel cells anywhere from several watts to multiple megawatts.

Types of Fuel Cells

The main difference among fuel cell types is the electrolyte and the difference in startup time.

  • Proton Exchange Membrane Fuel Cell (PEMFC): Fast start fuel cells can deliver power within 1 second and typically rely on a proton-conducting polymer membrane.
  • Phosphoric Acid Fuel Cell (PAFC): Phosphoric acid fuel cells were first introduced in 1961. Phosphoric acid is used as the electrolyte and these cells commonly work in temperatures of 150 to 200 degrees Celsius.
  • Solid Oxide Fuel Cell (SOFC): This type of fuel cell uses a solid material, most commonly a ceramic material, as the electrolyte. Because SOFCs are solid, they are not limited to the flat plane configuration of other types of fuel cells and are often designed as rolled tubes. They require high operating temperatures (800–1000 °C) and can be run on a variety of fuels.
  • Molten Carbonate Fuel Cell (MCFC): Molten carbonate fuel cells (MCFCs) require a high operating temperature, 650 °C, similar to SOFCs. MCFCs use lithium potassium carbonate salt as an electrolyte, and this salt liquefies at high temperatures, allowing for the movement of charge within the cell – in this case, negative carbonate ions.

Fuel Cell Efficiencies

The energy efficiency of a fuel cell is generally between 40–60%, or up to 85% efficient in cogeneration if waste heat is captured for use.

DescriptionEfficiency
Proton Exchange Membrane Fuel Cell40%
Phosphoric Acid Fuel Cell40%
Solid Oxide Fuel Cell45-60%
Molten Carbonate Fuel Cell52-65%

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